Zettelkasten (German for “slip-box”) is a flexible note-taking methodology created by Niklas Luhmann. A Zettelkasten (or slip-box) refers to a collection of literature notes (optional) and permanent notes, which are formed from fleeting notes.
Notes in the slip-box are not organized by any form of hierarchy, because humans don’t think in terms of hierarchy, but connections. This is why notes in the slip-box are interconnected by bidirectional links rather than organized under hierarchies or categories. Thanks to this feature, we can easily process and store notes, especially interdisciplinary ones, in a Zettelkasten. Such interconnection can also facilitate various creative tasks (i.e. writing, research, and brainstorming in general), since a slip-box user can easily branch from one note in a topic to dozens if not hundreds of related notes.
Despite the benefits of bidirectional linking, it is worth noting that a densely linked slip-box can also overcomplicate how we perceive reality ( Venkatesh Rao, 2016 podcast with Farnam Street). One way to combat this is to build maps of content to hierarchically index major notes, and also use your slip-box to produce writings that summarizes complex networks.